"Basically it finishes in a stadium and they say it’s full of 50,000 people," he explained.

"If you complete in less than four and a half hours you finish in the stadium and that’s what I’d like to do. You start in the stadium with a huge crowd as you depart and it’s 26.2 miles through the city."

Dunwoody, whose last marathon was in Dublin in 2004, is raising money for the Injured Jockeys' Fund and the Ebony Horse Club in Brixton, with a target of £2,500.